1. Field of the Invention
The present invention relates to a process for manufacturing an optical element and, more particularly, to a process for obtaining an optical element having an optical function surface directly from a molding material by press molding.
2. Related Background Art
In recent years, a process for manufacturing an optical element having a high-precision optical function surface which requires no post-working operations such as grinding and polishing has been developed. In this process, a material for molding an optical element, e.g., a glass blank which is preliminarily molded to have a given shape and surface precision is put in molds having a predetermined surface precision, and is press-molded to manufacture an optical element.
In this press molding process, upper and lower mold members are slidably arranged in a shell mold member to oppose each other, and a molding material is introduced in a cavity defined by these upper, lower, and shell mold members. As an atmosphere for preventing the mold members from being oxidized, a non-oxidizing atmosphere, e.g., nitrogen atmosphere is selected, and the mold members are heated to a molding enable temperature, e.g., until the molding material has a viscosity of 10.sup.8 to 10.sup.12 P. The mold members are then closed, and are pressed for an appropriate period of time, thereby transferring the surface pattern of the mold members onto the surface of the molding material. The temperature of the mold members is decreased to a temperature sufficiently lower than a glass transition temperature of the molding material, the compression pressure is released, and the mold members are opened. Thus, a molded optical element is removed.
Note that a molding material may be preheated to an appropriate temperature before it is introduced into the mold members, or it may be heated to a molding enable temperature, and then may be introduced into the mold members. Furthermore, a molding material may be conveyed together with the mold members, so that heating, pressing, and cooling are performed at predetermined locations, thereby attaining continuous, high-speed operations.
The above-mentioned process for press-molding an optical element, and the apparatus therefor are disclosed in, e.g., Japanese Laid-Open Patent Application Nos. 58-84134 and 49-97009, GB Patent No. 378199, Japanese Laid-Open Patent Application Nos. 63-11529, 59-150728, and 61-26528, and the like.
In the press molding process for an optical element, as described above, a molding material and the mold members are often welded to each other. Once they are welded to each other, a molded element may be cracked due to a tensile force applied to the element based on a difference between the thermal expansion coefficients of the molded optical element and the mold members during cooling after molding, or when the molded optical element is removed from the mold members, a portion of the element may be attached to and left on the mold members, thus adversely influencing the molding operations.
In order to prevent welding, a parting agent may be used. However, as for an optical element, in particular, an optical function surface is required to have high smoothness. Thus, the use of the parting agent leads to degraded quality, e.g., a decrease in smoothness, a decrease in transparency, and the like when the parting agent remains. Therefore, it is not preferable to use the parting agent.
Materials of mold members which do not easily cause welding have been studied. However, prevention of welding by selecting materials of the mold members narrows a selection range of the materials of the mold members, resulting in disadvantages.